1. Field of the Invention
The present invention relates to a separator of a proton exchange fuel cell using solid polymer membrane as electrolyte and its manufacturing method, and more particularly to a separator and its manufacturing method for providing a separator of a proton exchange fuel cell in compact size and light weight at low cost.
2. Description of the Related Art
A fuel cell is a device to convert chemical energy of fuel into electric energy by bringing fuel such as hydrogen and oxidizing agent such as air to electrochemically react each other.
Various types of fuel cells which differ according to type of electrolyte used are known; for instance, phosphoric acid type, fused carbonate type, solid oxide type and proton exchange type. Of these fuel cells, a proton exchange fuel cell is a fuel cell utilizing the fact that when polymer resin membrane containing proton exchange radical in module is saturated with water it acts as proton conductive electrolyte. The proton exchange fuel cell acts in a relatively low temperature range with excellent power generating efficiency and attracts attention in recent years.
FIG. 9 is a diagram showing the structure of a unit cell that is a base unit of a conventional proton exchange fuel cell.
As shown in FIG. 9, a unit cell 1 is composed of an ion conductive solid polymer membrane 2, an anode electrode 3 and a cathode electrode 4 arranged with solid polymer membrane 2 interposed between them. Further, at the outsides of these electrodes 3, 4, there are arranged an anode electrode side separator 5 and a cathode electrode side separator 6, each of which is gas impermeable and has a gas supply groove for supplying reaction gas to one of electrodes 3, 4.
As ion conductive solid polymer membrane 2, for instance, perfluorocarbon-sulfonic acid (Nafion R: Du Pont, U.S.A.), which is a proton exchange membrane, is known. Solid polymer membrane 2 contains hydrogen ion exchange radical in its molecule, and functions as an ion conductive electrolyte when saturated in water, and also, has a function to separate fuel 7 supplied from anode electrode 3 side and an oxidizing agent 8 supplied from cathode electrode 4 side.
Anode electrode 3 arranged at one side of solid polymer membrane 2 is formed of a catalytic layer 3a and a porous carbon flat plate 3b. Further, cathode electrode 4 arranged opposing anode electrode 3 is formed of a catalytic layer 4a and a porous carbon flat plate 4b. 
Separator 5 at the anode electrode side is composed of a separator substrate 9 and fuel supply grooves 10a, 10b arranged at both sides of separator substrate 9 for supplying fuel.
On the other hand, separator 6 arranged at the cathode electrode side is composed of a separator substrate 11, an oxidizing agent supply groove 12 for supplying an oxidizing agent arranged on one surface of separator substrate 11 at the surface side contacting cathode electrode 4, and a fuel supply groove 10 for supplying fuel arranged on another surface of separator substrate 11.
The principle of unit cell 1 will be described below.
When fuel 7 is supplied to anode electrode 3 and oxidizing agent 8 is supplied to cathode electrode 4, the electromotive force is generated by the electrochemical reaction between a pair of electrodes 3, 4 of unit cell 1. Normally, hydrogen is used as fuel 7 and air is used as oxidizing agent 8.
When hydrogen is supplied as fuel to anode electrode 3, hydrogen is ionized into hydrogen ion and electron in anode catalytic layer 3a (Anode reaction). The hydrogen ion moves to cathode electrode 4 through solid polymer membrane 2, and the electron moves to cathode electrode 4 through an external circuit. On the other hand, the oxygen contained in the air is supplied to cathode electrode 4 as oxidizing agent 8 causes the cathode reaction by the hydrogen ion and the electron in catalytic layer 4a to generate water. At this time, the electrons pass through the external circuit and become a current and is able to feed electric power. In other words, in anode electrode 3 and cathode electrode 4, reactions shown below will progress. Further, the generated water is discharged together with not-reacted gas to the outside of unit cell 1.    Anode Reaction: H2→2H++2e−    Cathode Reaction: 2H++1/202+2e−→H2O
In such unit cell 1, if water content in solid polymer membrane 2 becomes less, ion resistance becomes high, and mixing of fuel 7 and oxidizing agent 8 (crossover) is taken place, and unit cell 1 is not able to generate the electric power. So, it is desirable to keep solid polymer membrane 2 in the state saturated with water.
Further, when the hydrogen ion ionized in anode electrode 3 at the power generation moves to cathode electrode 4 through solid polymer membrane 2, water also moves jointly. So, at the anode electrode 3 side, solid polymer membrane 2 tends to become dry. Further, if moisture contained in supplied fuel 7 or supplied air is less, solid polymer membrane 2 tends to become dry at around respective inlet ports of reaction gasses. For this reason, pre-humidified fuel 7 and pre-humidified oxidizing agent 8 are generally supplied to unit cell 1.
By the way, electromotive force of unit cell 1 is low as below 1 volt, and a cell stack is generally formed by laminating several tens to several hundreds of unit cells 1 via separators 5, 6 arranged at the upper and lower sides of unit cells 1. Cooling plates are inserted into respective unit cells 1 in order to control the temperature rise of the cell stack resulting from the power generation.
Separators 5, 6 used in a proton exchange fuel cell are required to be impermeable to reaction gas and cooling water in order to provide with the function to separate each of unit cells 1. On the other hand, separators 5, 6 are also required to be electrically conductive in order to laminate unit cells 1 to provide a cell stack and to function as the fuel cell. Normally, a proton exchange fuel cell is operated at relatively low temperature of 70˜90° C. Separators 5, 6 inside the proton exchange fuel cell are under the severe environment where they are exposed to the air containing water vapor whose vapor pressure is close to a saturated vapor pressure at the temperature of 70˜90° C., and at the same time, potential difference is generated between separators 5, 6 pursuant to the electrochemical reaction. So, it is necessary to select a corrosion proof material for the separators 5, 6. As corrosion proof material, stainless steel, etc. are generally used. When stainless steel, etc. are applied to separators 5, 6, the surface thereof is oxidized and passive state membrane is formed on the surface thereof. As a result, resistance loss of the fuel cell becomes large and power generating efficiency drops largely.
In the U.S.A., during 1970's, for the separators of the proton exchange fuel cell developed for the space shuttle, niobium which is excellent corrosion proof noble metal, was used. However, noble metal materials have such defects that they are extremely expensive and heavy. So, as disclosed in U.S. Pat. No. 5,521,018, Ballard Power Systems Inc. of Canada uses carbon plates for separators so as to reduce the weight and cost of a cell stack.
FIG. 10 shows the construction of a cell stack of a conventional proton exchange fuel cell using carbon plates for separators.
As sown in FIG. 10, a cell stack 13 is composed of, in an outer frame 14, a cell portion 15 which generates electric power by reacting gas, and a humidifying portion 16 for humidifying reaction gas. In cell portion 15, a plurality of unit cells 1 are arranged in outer frame 14.
FIG. 11 is a schematic diagram showing the structure of conventional unit cell 1 in cell portion 15 shown in FIG. 10.
As shown in FIG. 11, in unit cell 1 arranged in cell portion 15 is in the structure as described below. Anode electrode 3 and cathode electrode 4 are arranged with ion conductive solid polymer membrane 2 interposed between them. A cooling separator 17 is provided at the outside of anode electrode 3, and anode electrode side separator 5 is provided at the further outside of cooling separator 17. Further, at the outside of cathode electrode 4, cathode electrode side separator 6 is provided.
Cooling separator 17 is provided to prevent the heating of the cell portion 15 by absorbing the reaction heat generated from the reaction by cooling water.
FIG. 12 is a plan view showing the construction of conventional separator 6 provided at the cathode electrode side.
As shown in FIG. 12, separator 6 is composed of a nearly square shaped separator substrate 11 made of a carbon plate with an air induction port 18 for inducing air and a fuel gas induction port 19 for inducing fuel gas provided at one corner of separator substrate 11. At the side opposing these induction ports 18, 19, an air discharging port 20 for discharging air and a fuel gas discharging port 21 for discharging fuel gas are provided. Further, at other corners of separator substrate 11, a cooling water induction port 22 and a cooling water discharging port 23 are provided. On separator substrate 11, a serpentine shaped air groove 24 is formed for inducing air to the reaction surface. Air groove 24 connects air induction port 18 and air discharging port 20. Though not shown in FIG. 12, at the under side of separator substrate 11, the fuel supplying groove is formed, which connects fuel gas induction port 19 and fuel gas discharging port 21.
Air groove 24 is formed on separator substrate 11 made of relatively soft carbon plate by applying the press working at one surface thereof. Further, the fuel supplying groove is formed on separator substrate 11 by applying the press working at another surface thereof.
Further, although not illustrated here, separator 5 at the anode electrode side and cooling separator 17 have almost the same structure as that of separator 6 at the cathode electrode side.
The structure of humidifying portion 16 shown in FIG. 10 is almost the same as that of cell portion 15. In cell portion 15, reaction gasses mutually contact via solid polymer membrane 2. However, in humidifying portion 16, air or fuel gas, that is reaction gas, is humidified by contacting cooling water via a steam transmission membrane.
However, even in the case of separators 5, 6 as described above, there is still a restriction for making the thickness of separators 5, 6 thin.
As reasons for this, it is pointed out that first, in the case of a proton exchange fuel cell using a carbon plate for separators 5, 6, a certain thickness is needed for the carbon plate in order to maintain the strength as separators 5, 6. Secondly, a carbon plate is intrinsically porous material and it is necessary to prevent transmission of gas and water between the separators, and therefore, it is restricted to make the thickness of separators 5, 6 thin. In the U.S. Pat. No. 5,521,018, the thickness of separators is 1.6 mm and a certain thickness is demanded.
To make cell stack 13 compact, it is most important to make the thickness of unit cell 1 thin. However, when a carbon plate is applied to a separator, there is such a problem that, it is restricted to make the separators thin and it is difficult to make cell stack in a compact size.
Furthermore, as the carbon material itself is expensive, there is such a problem that it is difficult to provide a cell stack 13 at low cost.
Furthermore, as the carbon plate has lower thermal conductivity than that of such metals as aluminum, copper, etc., it is needed to cool down unit cells 1 by inserting cooling plates 17 to flow cooling water between respective unit cells 1. Accordingly, there is such a problem that a cell stack becomes a larger size, and air-cooling is difficult to adopt in the proton exchange fuel cell.
On the other hand, in the case wherein metal is used as separator substrate instead of carbon plate, there is such a problem that the corrosion is generated when using the separators in the saturated steam atmosphere or by the potential difference generated specifically to the fuel cell and thereby the cell performance drops.